The invention relates to a method and an apparatus for controlling the operation of a Diesel engine. More particularly the invention relates to a method and an apparatus for controlling the onset of fuel injection in Diesel engines so as to obtain optimum operation in dependence on external requirements over the entire operational domain of the engine. The method and apparatus of the invention further provide the selection of an optimum operational point and a set-point signal related thereto which is compared with the actual injection timing in order to provide a corrective signal.